My simcraft actionlist shows that waiting for fiend no more than 35 seconds is Ok (still an increase of about 1K DPS). So I wouldn't blanket say waiting for cooldowns is an automatic DPS loss. There is also some encounters were the DPS outside of specific phases is irrelevant, and of course saving your cooldowns for these phases is perfectly ok.
The problem with using MS/MB under bloodlust/heroism is the fact it brings the cast time of the spell under the GCD. Most spriests have 30% from gear already, +5% raid buff + darkness and the fact haste is caculated multiplicatively means we are already quite close to the 50% required to GCD cap (40.5%). Getting such a large haste buff means that about only 8-9% of it is effectively used which means you are only using ~25% of it's effect.
For those of you who aren't aware there is a hard cap on the GCD of 1 second. That means haste will reduce the GCD to that level but no further. So if a spell takes 0.5sec to cast you are still locked out of casting for 0.5 seconds while the GCD refreshes. So reducing the cast time of a spell from 1.1 seconds to 0.5 seconds only gives you a benefit of 0.1 seconds (not the 0.6 as you might expect).
The other issue you will have is that MS will cast too quickly to line up nicely with fiend melee swings, resulting in some MS casts with 0 orbs.
For all of these reasons I wouldn't use MS/MB under any large haste proc/buff. It makes it not very good for early madness platforms, your normal cast priority should yield better DPS.